The invention relates to electrolytic cells whose cathode comprises a vertical flow of mercury. Such electrolytic cells have already been used, for instance in the preparation of chlorine. The cathode consists of a film of mercury which flows along a vertical wall. While such an electrolytic cell is advantageous, since the horizontal surface required for locating it is small, it has attendant drawbacks: the area of cathode surface per unit flow of mercury is low.
The inventory of mercury used in these installations is consequently large and at least as great as those used in "horizontal" electrolysers. Moreover, a risk of contamination of the electrolytes exists if the wall which supports the mercury film contains metal.
In another prior art electrolyser (French Pat. No. 352,029), the mercury is received in channels located above each other and runs downwardly by overflows. Then, the cathode surface essentially consists of the free level of mercury in the channels and the ratio between the surface area and the inventory of mercury is low.
It is an object of the invention to provide an improved vertical electrolytic cell, having an increased active cathode area for a given amount of mercury inventory.
It is another object of the invention to provide a vertical electrolytic cell in which the risk of contamination is substantially reduced.
It is a more specific object of the invention to provide an electrolytic cell which is particularly adapted to the reduction of uranium ions in an aqueous solution.
According to an aspect of the invention, there is provided an electrolytic cell having cathode means and anode means, wherein said cathode means comprises at least one continuous thread of mercury flowing down by gravity from an aperture in the bottom wall of at least one channel.
Generally, the electrolytic cell comprises a casing and vertical diaphragm means separating an anode compartment locating said anode means having a vertical surface and a cathode compartment locating said cathode means in said casing.
The vertical diaphragm may be impervious to liquids and gases, and permeable to ions. Any ingress of one electrolyte into the other is then prevented. However, pervious diaphragms may also be used.
The size of the apertures in the bottom of the channel or channels should be selected for the mercury threads to remain continuous, at least when a voltage is applied to the electrolytic cell. Typically, for the mercury threads to remain continuous, they are given a diameter which does not exceed about 5 mm and their free length should not exceed about 15 cm from the outlet aperture up to the channel or that which receives them. It is of interest to note that the mercury thread becomes fractioned in droplets when no voltage is applied to the electrolytic cell, due to the oxidation of mercury. Oxidation is not present any longer when electrolysis occurs. From a first consideration, it would consequently appear that it would be impossible to carry out the invention, because it apparently leads to use mercury threads having such a diameter that the approach would be irrealistic.
It appears clearly that the active surface of the cathode is substantially increased with respect to that of a mercury sheet running down a metal support and the risk of pollution is removed.
According to a first embodiment of the invention, the electrolytic cell is rectangular and comprises an anode compartment in the form of a parallelepiped filled with anolyte and equipped with an anode, a plane diaphragm and a cathode compartment in the form of a parallelepiped filled with catholyte, through which compartment descend continuous streams of mercury constituting the cathode. The electrolyser may comprise a plurality of anodic and cathodic parallelepipedal compartments separated by plane diaphragms and arranged side by side in the manner of the cells of a filter press.
According to a second embodiment, the electrolytic cell is cylindrical and comprises an anode compartment filled with anolyte and equipped with a tubular anode, a tubular diaphragm and an annular cathode compartment filled with catholyte through which flow the continuous streams of mercury which constitute the cathode. Again, an electrolyser may comprise a plurality of anode and cathode compartments separated by diaphragms. Typically, a single, large anode compartment is provided, which is filled with anolyte in which a plurality of units are assembled, each comprising a tubular anode, a tubular diaphragm and an annular cathode compartment filled with catholyte.
When an electrolyser has several cells, the electric connections may be either in series or in parallel or in series parallel.
The catholyte may be supplied either in series or in parallel but each compartment has its own independent circulation of mercury for preventing short-circuits.
In order to maintain electrolysis, the streams of mercury which form the cathode must flow continuously so that the path of electric current will not be cut off. The length of the mercury threads and the size of the apertures (which are preferably, but not necessarily circular in shape) are determined in dependence of various factors, in particular the voltage applied to the terminals of the electrolytic cell and the nature, concentration and rate of flow of the electrolytes.
The or each cathode compartment may contain a plurality of channels fixed one above the other to a support, the highest channel being supplied with mercury. Each channel has apertures in its bottom wall through which the continuous streams of mercury flow. Each channel, except the first, is supplied with the continuous stream of mercury from the channel above it.
The channels may be secured to a support made of a conductive material having a sufficient overvoltage (for example graphite). This arrangement results in an electrolytic cell with a double cathode, a mercury cathode formed by the continuous streams of mercury and a second cathode consisting of the conductive support.
The channels may be made of a conductive material and connected to the electric supply. Alternatively, they may be made of an insulating material, in which case means must be provided to conduct the electric current to the mercury contained in them. The choice of materials used for the electrolytic cell, the anode, the compartments, the channels, the diaphragm and the electric connections will depend on the results to be obtained and the nature of the compounds to be treated.
The anodes and/or the supports for the channels may have internal cavities located in the path of a cooling fluid, for example water. This arrangement enables the electrolytic cell to be cooled in situ so that the external heat exchangers otherwise required for cooling the electrolytes and mercury can be dispensed with and hence the quantity of mercury used can be further reduced. The internal cavities may as well or in addition locate pumps for the circulation of mercury and/or of the electrolytes.